Process of absorbing nitrous gases.



' H. PAULING.

PROCESS 0F ABSORBING NITROUS GASES. l

APPLICATION FILED JUNE s, 1911A Patented sept. 10,1912.

- UNITEE sTATEsgENT OFFICE.

HARRY PAULING, F GELSENKIRCHEN, GERMANY, ASSIGNOR T0 THE FIRM OF SAL- PETERSUREJNDUSTRIE-GESELLSCHAFT, G. M. B. H., 0Fv GELSENKIRCHEN, GERd MANY.

PROCESS Glli ABSORBING NITROUS GASES.-

T9 all whom it 'may concern Be itknovvn that I', HARRY PAULING, a subject ofthe King of Prussia, residing at Gelsenkirchen, in the Kingdom of Prussia and German Empire, have invented a new and useful Improved Process for Absorbing Nitrous Gases, of which the following is a Y sulfuric acid, are next deprived of a portion of the moisture taken up from the dilute sulfuric acid, are further dried by means of concentrated sulfuric acid, are next treated by a. Wet absorbing process, as by treatment with water, whereby nitrogen oxids are largely removed from said gases with the formation of nitric acid, and are finally treated with concentrated sulfuric acid whereby the remaining and ditiicultly ab# sorbable nitrogen oxids are substantially completely removed from said gases, the nitrated sulfuric acid from this final step being employed to dry the ases at au earlier stage of the process there )y undergoing diiution and denitration, the dilute sulfuric acid thus obtained being employed in the first treating step and thereby being reconeentrated, and the sulfuric acid thus reconcentrated being passed repeatedly through the cycle of steps just described; all as more fully hereinafter set forth and as claimed.

Heretofore it has not been possible to quantitatively absorb-that is absorb without any loss-the nitrogen oxids in mixtures of gases resulting from the combustion of air, because in the usual wet absorbing processes, as for example where the gases containing oxids are treated With Water, a part ofthe NO2 produced in the combustion of the air is again decomposed in the absorp-v tion process into NO and is consequently not absorbed. Nitrogen peroxid, (NO2) vreacts with Warm Water to yield nitric acid (HNOQ and nitric oxid (N0); and since nitric oxid is only slightly soluble lin water, very little of it is retained in solution. On the other hand, as in the production of nitrate of 1imefor example, a quantitative ab- Specication of Letters Patent.

Application led .Tune 8, 1911.

Patented Sept. 10, 1.912. Serial No. 632,076.

sorption by an alkaline treating solution results in the formation of an excess of nitrite which cannot be utilized for many purposes.

The present applicant has now discovered that the nitrous gases'can be so quantitatively absorbed as to form nitric acid if the unabsorbed nitrous gases, -hereinafter vreferred toast'the residual gases, which leave the wet absorbing apparatus proper, after being suitably oxidized, are taken up by sulfuric acid. These residual nitrous gases are usually accompanied of course by varying quantities of free nitrogemoxygen, and the like.

In order to avoid a separate denitration of the sulfuric acid after it has absorbed the residual gases, the following method .is adopted. The gases leaving the reaction furnace at a very high temperature and coutaining nitrogen oxids resulting from the heat treatment of air or other gaseous mixture of nitrogen and oxygen, after their seni sible heat has been utilized for raising steam or for other purposes, are conducted at a temperature of about 400O (l. or more into an acid proof tower to which dilute sulfuric acid is supplied for concmiti'ation. ln this tower, the hot gases abstract water from the dilute sulfuric acid. The concentrated sul` furic acid leaving this tower is used for absorbing the residual gases before mentioned. The gases which leave the sulfuric acid Acouccntrating tower are of course strongly charged with moisture, and they are accordingly cooled down to the usual temperature by suitable apparatus, the greater part of the water taken upy from the sulfuric acid beingthus separated, together with mole or less nitric acid. Nevertheless these gases a fter cooling still contain so much moisture that, as a consequence, the concentration of the nitric acid produced in the following absorbing process would be much reduced ifI further measures were not. taken for removmg moisture. ln order to overcome this dif! iiculty the gases are caused to come into coutact with a countercurrent of sulfuric acid in a drying tower or other similar appara-4 tus. The sulfuric acid employed here comes from an apparatus in which the residual gases are absorbed andthercfoie contains compounds of nitrogen oxids with sulfuric acid such as nitrosyl sulfuric acid and the like. The moist nitrous gases are thus dried while, `at the same time, the nitrated sulfuric acid is diluted and heated by taking up water vapor, the total latent heat of the vater vapor being liberated. By this dilution and heating the sulfuric acid is dcnitrated to such an extent that only a small denitrating plant is necessary, in order to expel, if desired, such traces of nitrous gases as may still be present in the acid.

The dilute and largely denitrated sulfuric acid flowing out of the drying apparatus is delivered to the concentratitni tower again and repeats the circulation just described. Prior to its deliveryl to the concentration tower, the dilute sulfuric acid may be completely denitrated by special means where such denitration desirable. This method of working has the further considerable advantage that the mixed gases lea ving the drying tower on coming into contact with the relatively dilute nitric acid of the first absorption tower, wl 'le giving up a large proportion of their content of nitrogen oxids to the absorbing liquid, become partly saturated with moisture, and therefore tend to concentrate the nitric acid of the firsttower. liurthermore, the gases are well cooled down by this absorption and this leads to a higher concentration in the first tower and also to considerably better ab sorption, because, according to the experience of the applicant, the Wet absorption takes place more quickly and in smaller apparatus in proportion to the extent to which the nitrous gases and the absorbing liquid are cooled.

In the accompanying drawing I have shown, diagrannnatically, a plan of apparatus suitable for use in the presenti process.

The nitrous gases produced in the apparatus a, an electric furnacefor example, deliver the greater part of their heat to the boiler b and cnte at a temperature of 4000 C. or more into the acid proof tower Here the gases absorb moisture from dilute sulfuric acid moving in counter-current, so that they become strongly charged with moisture. lt is therefore necessary to remove this moisture from the gases which is done partly in the condensing or cooling plant d connected with the tower The gases are then further dried in the drying tower c. ln this tower the gases are brought into intimate contact ith concentrated sulfuric acid moving in countercurrent so that they are dried, whereupon they are first oxidized in the tower f as by treatnient with air or otherwise, and are then subjected in the towers g to a wet absorption. The remaining nitrogen oxide in the gases issuing from the towers y are entirely absorbed by highly concentrated sulfuric ,acid in the tower It.

rlhe sulfuric acid circulates as follows: `The-sulfuric acid leaving the lower end of the tower it in which the remaining onids Loaf/gew are absorbed by highly concentrated sulfuric acid conta-ins compounds of nitrogen oxids with sulfuric acid, such as nitrosyl sulfuric acid, and is conducted by the pipe to the drying tower c, where it takes up the moisture contained in the moving in countercurrent thereteuand is consequently diluted and more or less completely denitrated. From thence the dilute sulfuric acid may pass through a` small denitration plant to the concentrating' tower c, in which it is concentrated and then conducted back by the pipe Z to the tower it, where the circulation begins again.

rlChe process carried out practically amounts therefore to treating the current of gases containing nitrogen oxids in a plurality of more or less separated stages'with counteriowing sulfuric acid, the character and coi'icentraticn of the sulfuric acid at each such stage being adapted to the requirements of the gaseous stream `at that stage. it an initial stage, the nitrous gases coming from an electric furnace, for example, and at a relatively high temperature, meet and progress against a stream of dilute sulfuric acid from which they abstract a large amount of moisture, being cooled somewhat in the operation. At an intermediate stage, after removal of some of the moisture taken up in the initial stage, the gases are dried by an oppositely flowing current of strong sulfuric acid, any nitro products which may be present in the sulfuric acid being largely eliminated at this stage and passing on with. the current of gases. I'lhe current of gases then passes through a trbin of absorption apparatus where the bulk of the nitrogen oxids is absorbed in water, for example, forming a solution of nitric acid. Any oxids of low solubility, such as nitric oxid resulting both from possibly insutlicient previous oxidation as weil as that evolved from the absorption reaction accompanying the solution of the more soluble oxide, of course escape absorption and pass on with the gaseous current '.lhese unabsorbed oxids or residual gases are recovered in a, tinal stage by treatment with highly ctnicentrated suln furie acid flowing in coinitercurrciit, nitric oxid dissolving' in or combining with such sulfuric acid to form what may be termed a nitrated sulfuric acid, probably containing compounds of the nitrosyl sulfuric type. As before explained, the sulfuric acid may be circulated in a closed circuit, contacting with the countertlowing current of gases at the various as retpiired, and undergoing f :yclically the variations in character and concetitration before indicated.

That l claim is l. The process of absorbing oxids of nitrogen, which comprises subjecting gases containing said oxids to a wet absorbing process, absorbing the oxide remaining in the gases by contact with sulfuric acid, and using` the nitrated sulfuric acid thus obtained for drying gases containing oxids of nitrogen previous to their subjection to the absorbing process.

'lhe process of absorbing nitrous gases, which consists in bringing said gases at a temperature of about 400o l. into contact with dilute sulfuric acid, condensing a part of the nioistln'e taken up by the gases from said dilute sulfuric acid, bringing the gases thus partially dried into contact with concentrated sulfuric acid to dry the saine, oxidizing said dried gases, subjecting said oxi l dif/,ed gases to a wet absorbing process7 and absorbin the oxids reinainin f in the. lrases n n by concentrated suluric acid.

3. In the manufacture of nitric acid, the process which comprises passing a current of gases containing nitrogen oxids through suitable apparatus, treating said current at an initial stage with dilute sulfuric acid, treating said current. at an intermediate stage with concentrated sulfuric acid containing nitrosyl sulfuric acid, and treating said current at a nal stage with concentrated and 'substantially pure sulfuric acid.

In testimony whereof l have signed my name to this specication in the presence of two subscribing Witnesses.

HARRY PAUL'ING. lVitnesses RICHARD Minuten, M. Krucrrnns. 

